Magnetic order in pyrochlore iridate Y$_2$Ir$_2$O$_7$ probed by electron paramagnetic resonance

We performed electron paramagnetic resonance (EPR) studies of the magnetic properties of the pyrochlore iridate Y$_2$Ir$_2$O$_7$ compound. The resonance line in the EPR spectrum shifts towards lower fields when the temperature is below 100 K, suggesting the appearance of a local field generated by the ordered magnetic moments at low temperatures. The temperature dependent local field and magnetization both show hysteresis between zero field cooling and field cooling, however, the magnetic ordering temperature $T_O$ ($\sim$ 100 K) determined by EPR is about 50 K lower than the transition temperature $T_M$ ($\sim$ 150 K) in the $M-T$ measurements, indicating a possible short-range order state at the intermediate temperature. A hyperfine structure was detected below the long-range ordering temperature, suggesting the co-existence of isolated paramagnetic Ir$^{4+}$ moments with the magnetic order state.